Separate bootstrap building logic into the envoy/bootstrap package

[steeling]

This PR separates the bootstrap config build logic into the bootstrap package. There are 0 "logic" code changes, only moving code around, exporting previously unexported functions, and prefacing methods/types with the new package name.

The main reason to do this is that there are changes coming with respect to rotating the secrets with the bootstrap config. Because of this, we will need to modify how we are creating and writing the bootstrap config.

Currently the code is tightly coupled with generating the config and writing it. This PR begins the process of decoupling, and brings us closer the the builder pattern we plan on implementing.

The main changes include:

1. Move HealthProbes, which are an internal construct, to a separate models package, so that it can be used by both the injector and the bootstrap builder.
2. Move all code that is involved in building the bootstrap config to the bootstrap package. A subsequent PR will make actual changes to create a logical builder.

[jaellio]

Can this be noted as part of #4839?

[steeling]

Can this be noted as part of #4839?

This would actually be part of #4818

[shalier]

LGTM