Added GitVersion to automatically increment package and assembly version

[ewingjm]

NO_CI the resulting commit of this PR needs to be tagged with the initial version before the CI pipeline can run

GitVersion has been configured in mainline mode. This means that it will automatically increment to a full version on each commit in master.

Using default version bump regex:

major-version-bump-message: '\+semver:\s?(breaking|major)'
minor-version-bump-message: '\+semver:\s?(feature|minor)'
patch-version-bump-message: '\+semver:\s?(fix|patch)'

Patch version incremented by default. For more info: https://gitversion.net/docs/more-info/version-increments

Pipeline has also been updated to pack the NuGet package using the version from GitVersion.

Note: we will need to stop using pre-release tags as mainline mode on GitVersion doesn't support it. I believe this is fine as, in SemVer, anything before the first major version is pre-release by definition