TransformBuilder::build takes &self intead of self

[rukai]

I think that semantically it makes a bit more sense to have a single build method that generates a transform, rather than cloning the builder and then building a transform by destroying the builder.

There should also be some performance improvements:

• Avoids having the two step process of clone -> build, this results in a bunch of extra copying on the stack which I doubt the compiler can optimize away because the structures are so deeply nested. (copies on the heap are unchanged because they only get copied during the clone)
• There are cases where we call TransformChainBuilder::build(&self) in a transform builder which results in needless internal clones due to taking an &self

[github-actions]

0 benchmark regressed. 1 benchmark improved. Please check the benchmark workflow logs for full details: https://github.com/shotover/shotover-proxy/actions/runs/4119075331

cassandra/protect_local_execute_unencrypted
                        time:   [569.91 µs 585.41 µs 603.23 µs]
                        thrpt:  [1.6577 Kelem/s 1.7082 Kelem/s 1.7547 Kelem/s]
                 change:
                        time:   [-36.938% -32.670% -27.782%] (p = 0.00 < 0.05)
                        thrpt:  [+38.471% +48.521% +58.575%]
                        Performance has improved.