luci-mod-network: Add HTTPS SVCB record en/decoding to DHCP

luci-mod-network: Add HTTPS SVCB record en/decoding to DHCP

This was a bit of code golf. Just for fun. Users seem to rely on HTTPS
record creation. The HTTPS record handling en/decodes compliant records
when tested against the RFC defined vectors, although there may exist
edge cases that are not compliant.

Escaped character encoding is not implemented (strict mode).

Any other arbitrary DNS record creation will be possible via the
addition of helper functions in this tools file.

Signed-off-by: Paul Donald <[email protected]>