Validate that SAM header tag keys are exactly 2 characters long

[jmarshall]

Description

As reported in #1477, the SAM specification (§1.3) requires that the keys of header tagged fields match /^[A-Za-z][A-Za-z0-9]$/, but HTSJDK even accepts keys that are not two characters in length.

This PR adds header tag key checking to SAMTextHeaderCodec's validation. It adds a new HEADER_TAG_INVALID_KEY entry to SAMValidationError, described as representing keys invalid due to either containing invalid characters or being the wrong length. This PR implements a length check but does not implement an alphanumeric character check.

You may prefer to either remove the future intention from the HEADER_TAG_INVALID_KEY documentation comment or to implement a character validity check (and add further test cases).

Things to think about before submitting:

• [x] Make sure your changes compile and new tests pass locally.
• [x] Add new tests or update existing ones:
  • A bug fix should include a test that previously would have failed and passes now.
  • New features should come with new tests that exercise and validate the new functionality.
• [ ] Extended the README / documentation, if necessary
• [x] Check your code style.
• [x] Write a clear commit title and message
  • The commit message should describe what changed and is targeted at htsjdk developers
  • Breaking changes should be mentioned in the commit message.

[nh13]

@cmnbroad any objection from your side?