Add --umi_tag option to output UMI as a SAM-style tag (e.g. XR:Z:UMI)

[yangmqglobe]

Implements the UMI tag output suggested in #226.

What

Adds a new optional --umi_tag=<TAG> parameter (e.g. --umi_tag=XR). When specified, the UMI is written as a SAM-style optional tag after the first field of the read name instead of being appended to the name:

# default (unchanged)
@V350398653L1C001R00100001764/1:TCGACC_GCGTAA

# with --umi_tag=XR
@V350398653L1C001R00100001764/1 XR:Z:TCGACC_GCGTAA

Notes

• The tag name is configurable (2-char SAM tag, e.g. XR, RX); --umi_prefix still applies inside the tag value (XR:Z:UMI_TCGACC_GCGTAA).
• --umi_delim continues to control only the default name-append mode; the _ separator between per_read/per_index UMI parts is unchanged.
• Tag mode is opt-in; default behavior is fully backward compatible.
• Validation rejects --umi_tag without --umi and malformed tag names.
• Unit tests added in UmiProcessor::test(); ./fastp test passes.