Adds a RuntimeImageResolver class and a corresponding new config variable LAMBDA_RUNTIME_IMAGE_MAPPING

Copied doc from the class:

Resolves Lambda runtimes to corresponding docker images
The default behavior resolves based on a prefix (including the repository) and a suffix (per runtime).

This can be customized via the LAMBDA_RUNTIME_IMAGE_MAPPING config in 2 distinct ways:

Option A: use a pattern string for the config variable that includes the "" string
e.g. "myrepo/lambda:-custom" would resolve the runtime "python3.9" to "myrepo/lambda:python3.9-custom"

Option B: use a JSON dict string for the config variable, mapping the runtime to the full image name & tag
e.g. {"python3.9": "myrepo/lambda:python3.9-custom", "python3.8": "myotherrepo/pylambda:3.8"}

  Note that with Option B this will only apply to the runtimes included in the dict.
  All other (non-included) runtimes will fall back to the default behavior.

sample configurations:

• LAMBDA_RUNTIME_IMAGE_MAPPING="customrepo/lambda-<runtime>:2022"
• LAMBDA_RUNTIME_IMAGE_MAPPING='{"python3.9": "customrepo/lambda-py-thon:3.9"}'