Add support to tapyrus-setup sign and createsig commands to sign xfield

[Naviabheeman]

This PR adds the command line support to generate a federation signature to make an xfield change.

-  tapyrus-setup blocks
-  tapyrus-setup sign
-  tapyrus-setup createsig

commands are used to create the xfield signature

Use XFieldHash type hash the serialized xfield to generate the signature
Add tests to verify signing xfield instead of block. A valid signature was generated using the commands in a 3 signer, 2 threshold setup and is added to the unit test.

[azuchi]

This change eliminates the effect of changing the threshold, so please make a test that also changes the threshold.

[Naviabheeman]

This is added in a new test test_load_federations_threshold_change too.

[Yamaguchi]

Will this line be executed?
I think this line is unreachable, as the same check is on line 204.

[Naviabheeman]

yes. this is unreachable. I'll fix it.

[Yamaguchi]

Are there any restrictions on the maximum or minimum MaxBlockSize?
If a small value (maybe 170bytes) is specified, will TapyrusCore reject the block?

According to https://github.com/chaintope/tapyrus-core/blob/master/doc/tapyrus/signedblocks.md, the minimum Block Header is 170byte.

[Naviabheeman]

Are there any restrictions on the maximum or minimum MaxBlockSize?

Tapyrus Core requires the minimum block size to be 1000 bytes. There is no maximum limit other than the 4 byte capacity allowed.

If a small value (maybe 170bytes) is specified, will TapyrusCore reject the block?

Yes, Tapyrus Core does not allow max block size in xfield to be set below 1000 bytes. Also, BlockAssembler in Tapyrus Core uses the same minimum limit. However if a smaller size valid block is created outside of Tapyrus Core like using tools like our test framework, there is no size restriction. The block is accepted by the node after validation.