Add version identification to the opentelemetry-proto tar archive

[marcalff]

Please add a VERSION file in the source repository itself, so it gets included in the opentelemetry-proto.tar.gz archive.

When using the *.tar.gz file produced in the release:

https://github.com/open-telemetry/opentelemetry-proto/archive/refs/tags/v0.19.0.tar.gz

the archive is named v0.19.0, sure, but there is no way (*) to inspect the content programmatically to confirm the code is for release 0.19.0

This is important, when someone wants to make a build using:
OPENTELEMETRY_PROTO_PATH=/path/to/opentelemetry-proto (where "a" tar.gz has been installed ... which one ?)
instead of pulling from github with a tag.

Please make the file easy to parse by makefiles, for example:

OPENTELEMETRY_PROTO_VERSION_STRING=0.19.0
OPENTELEMETRY_PROTO_VERSION_MAJOR=0
OPENTELEMETRY_PROTO_VERSION_MINOR=19
OPENTELEMETRY_PROTO_VERSION_PATCH=0

Please adjust RELEASING.md accordingly, so this version is maintained up to date.

(*) Parsing the change log is not an option here.

[tigrannajaryan]

Is this a common practice with repos?

@open-telemetry/specs-approvers thoughts?