Add type parameter for Command interface

[kwin]

Strong typing of Command's response type
This closes #1272

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[kwin]

@olamy Is this backwards-compatible enough? It won't affect consumers as ScmProvider does not use this. I consider this a SPI only interface (so only https://github.com/olamy/maven-scm-provider-svnjava would be affected).

[elharo]

can you resolve the conflicts?

[kwin]

can you resolve the conflicts?

Only once I have someone's confirmation that this is something we can/should do in a minor version upgrade.