gRPC swift package depends on [boringSSL-SwiftPM](https://github.com/firebase/boringSSL-SwiftPM), which hasn't been updated since more than two years ago (https://github.com/firebase/boringSSL-SwiftPM/pull/8).
Updating it is not trivial as the `Package.swift` file is maintained manually, a recent [upgrade to swift 5.5](https://github.com/firebase/boringSSL-SwiftPM/pull/16) took good amount of time only to repeatedly exclude one (sometimes two) non-code files and retry build to get new errors, until the build succeeds.
This PR uses the gRPC build system to generate the swift package file from the same boringssl version in third_party.
Will use the output to update the boringSSL-SwiftPM repo after this PR is merged.
Tested with
```
cp BoringSSL-Package.swift Package.swift
swift build --target openssl_grpc
swift test
```
Closes#36062
Update LICENSE text to 2015, implement a checker that validates
whether a 2014 or 2015 version of the license is on each {.c,.cc,.h} file.
Fix the ones that are missing it.
-) Detecting system and embedded presence of zlib and OpenSSL with ALPN.
-) Automatically disabling secure targets if no OpenSSL present
--> make all won't work if no OpenSSL is present, forcing the users to select the nonsecure Makefile targets explicitely.
-) Removing build instructions for OpenSSL - this isn't really necessary anymore.
-) Adding more blurb about OpenSSL and the new Makefile features.
Change on 2014/12/12 by nnoble <nnoble@google.com>
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82020890
Hannah Shi
Jan Tattermusch
Craig Tiller
nnoble