Create a simple checksum Maven plugin

### New feature, improvement proposal

Resolver 2.x:
* has extensible checksum SPI
* provides everything we need to create, verify them
* so this would be really a simple, dependency-less (resolver and maven plugin api provided) to create, verify checksums, and, if user extends Checksums via SPI, those become automatically available to this plugin

Proposed mojos:
* `checksums:create` - creates and attaches checksums for project artifacts
* `checksums:file-verify` - creates and displays checksums for provided file
* ?

Start by reviewing the Resolver 2.x checksum SPI and the Maven plugin API, then compare the proposal's checksums:create and checksums:file-verify mojos with existing plugin structure. Done means a dependency-less plugin can create and attach project-artifact checksums, and create and display checksums for a supplied file, including SPI-provided checksum types.

Written by the indexing model from the issue text.