build_verity_metadata: Support --signer_args argument.

build_verity_metadata: Support --signer_args argument.

Use argparse to parse input parameters for future extension.

Add 'build' and 'size' as two sub-commands.
- 'build_verity_metadata.py -s SIZE' becomes
  'build_verity_metadata.py size SIZE'.

- 'build_verity_metadata.py BLOCKS METADATA_IMAGE ...' becomes
  'build_verity_metadata.py build BLOCKS METADATA_IMAGE ...'.

The 'build' command now accepts an optional argument '--signer_args' to
specify signer specific arguments.

Bug: 31500665
Test: Building and signing work w/ and w/o --signer_args.

Change-Id: I3cfd2b584efad216b91d5fcd0314f4468ff7450f
(cherry picked from commit 39d1756abe66538a6cbdfc4c7b5377ce279e332c)