xds: support env variable GRPC_XDS_BOOTSTRAP_CONFIG for bootstrap config content

[howardjohn]

Currently, GRPC_XDS_BOOTSTRAP points to a file. In some cases, it much easier to stick the entire configuration in an env var than configure both an env var and a file. For example, with docker, its a simple --env for the former, and both --env and --volume (as well as maintaining the folder, potentially many bootstraps for many containers).

Some other motivations for this can be found in https://12factor.net/config.

[howardjohn]

Somewhat related to #4124

[srini100]

Thanks for the suggestion. Also related to grpc/grpc-java#7605 (comment).

We'll add this in all supports languages.