Cherry-pick sonic-net#21890

<!--
Please make sure you've read and understood our contributing guidelines;
https://github.com/sonic-net/SONiC/blob/gh-pages/CONTRIBUTING.md

Please provide following information to help code review process a bit
easier:
-->
### Description of PR
<!--
- Please include a summary of the change and which issue is fixed.
- Please also include relevant motivation and context. Where should
reviewer start? background context?
- List any dependencies that are required for this change.
-->

Summary:
This PR is to improve existing BGP tests to support BGP confederation.
Below two tests are improved
-
[test_bgp_peer_shutdown.py](https://github.com/sonic-net/sonic-mgmt/compare/fix_bgp_confed_test?expand=1#diff-9d72d8d9e2fcf1b7573aadae84686354be322286dfce64199d0b158d8002cda0)
-
[test_bgp_update_timer.py](https://github.com/sonic-net/sonic-mgmt/compare/fix_bgp_confed_test?expand=1#diff-85d6c9d73bdee5ce2f7bca42ac67428028046115c26e6223a0df78e8706e3bd6)

### Type of change

<!--
- Fill x for your type of change.
- e.g.
- [x] Bug fix
-->

- [ ] Bug fix
- [ ] Testbed and Framework(new/improvement)
- [ ] New Test case
    - [ ] Skipped for non-supported platforms
- [x] Test case improvement


### Back port request
- [ ] 202205
- [ ] 202305
- [ ] 202311
- [ ] 202405
- [ ] 202411
- [ ] 202505
- [x] 202511

### Approach
#### What is the motivation for this PR?
This PR is to improve existing BGP tests to support BGP confederation.

#### How did you do it?
1. Improve
[bgp.py](https://github.com/sonic-net/sonic-mgmt/compare/fix_bgp_confed_test?expand=1#diff-a6848e9239af6d197528a7960ad94553f237dc318bd195837d5a31a210ff879c)
to support using `vtysh` to configure BGP neighbors
2. Improve
[multi_asic.py](https://github.com/sonic-net/sonic-mgmt/compare/fix_bgp_confed_test?expand=1#diff-9b70276a3c2fb324d2737f57ab8ae139d7eb30cc886d3717f185088a9f8f241a)
to add a helper function to retrieve BGP confederation ASN.

#### How did you verify/test it?
Both tests are verified on LT2/FT2 testbeds.
```
collected 2 items                                                                                                                                                                                                                                              

bgp/test_bgp_update_timer.py::test_bgp_update_timer_single_route[default]  ^HPASSED                                                                                                                                                       [ 50%]
bgp/test_bgp_update_timer.py::test_bgp_update_timer_session_down[default] PASSED                                                                                                                                                       [100%]

collected 1 item                                                                                                                                                                                                                                               

bgp/test_bgp_peer_shutdown.py::test_bgp_peer_shutdown[default]  ^HPASSED                                                                                                                                                                  [100%]
```
#### Any platform specific information?
No.

#### Supported testbed topology if it's a new test case?
Not a new test.
### Documentation
<!--
(If it's a new feature, new test case)
Did you update documentation/Wiki relevant to your implementation?
Link to the wiki page?
-->