[Java] Add VarCharWriter#write(String)

[asfimport]

For convenience, since the default method requires allocating an ArrowBuf and such. See PR#11982 (comment)

It's generated in ComplexWriters.java, we could add an if and special-case VarChar to have a helper method that uses setBytes instead.

Reporter: David Li / @lidavidm

_{Note: This issue was originally created as ARROW-15907. Please see the migration documentation for further details.}

[rtadepalli]

I don't think this is possible because the AbstractFieldWriter is extended by all writer interfaces. AbstractFieldWriter extends FieldWriter, which extends ScalarWriter, which extends VarCharWriter. This makes it such that the helper new method must be implemented by all concrete writer implementations. We could make VarCharWriterImpl have this method, but it looks like the current code is set to program against the VarCharWriter interface, so I am not sure how it would end up working for one special case.

Please let me know if I am missing something.

[lidavidm]

VarCharWriter#writeString now exists (1eaa401)