Add deprecation warnings (#72)

Motivation:

gRPC Swift v2 has moved to a new repo, grpc-swift-2 and as a result this
package has a new major version. That's not all that discoverable.

Modifications:

Deprecate commonly used high-level types with a link to a forums post
explaining the move.

Result:

Users are notified about the move

Move to grpc-swift-2 (#70)

Motivation:

To support incremental migration, v2 has moved to the 'grpc-swift-2'
package.

Modifications:

- Update dependencies
- Update generated code
- Re-baseline availability
- Rename 'protoc-gen-grpc-swift' to 'protoc-gen-grpc-swift-2'

Result:

Easier to migrate from gRPC Swift 1 to 2

Move availability inline (#68)

Motivation:

It's hard for packages to incrementally adopt gRPC with platforms in the
package manifest as it requires packages to include platforms in their
manifest or mint a new major version.

Modifications:

- Move platform availability onto source code
- Check annotations in CI

Result:

Easier to adopt

Fix GoogleRPCStatus encoding/decoding (#58)

Motivation:

The 'rich' error model packs a google.rpc.status protobuf message into
the trailing metadata of an RPC. This should just be the base64 encoded
bytes of the serialzed message. At the moment this is packed within a
google.protobuf.any and then added to the metadata which doesn't interop
well with other languages.

Modifications:

- Remove the indirection
- Add methods for serializing/deserializing the RPC status wrapper

Result:

Better interop

Add command plugin to package products (#52)

Motivation:

The command plugin needs to be a product in order for it to be used.

Modifications:

- Add it to the products
- Give it the same name as the command, otherwise diagnostics (like
SwiftPM asking for permission to write to the package dir) will use a
different name which isn't obvious.
- Fix a few build warnings.

Result:

- Command plugin can be used

Relax dependency requirements (#43)

Bump dependency versions (#38)

Depend on beta 3 tag (#30)

Bump version (#24)

Bump grpc-swift version (#18)