rust: switch from OpenSSL to rustls

[wchargin]

Summary:
The RustBoard data server now uses rustls as a TLS backend instead of
using OpenSSL. This culls the libcrypto.so.1.1 and libssl.so.1.1
shared library dependencies, fixing #4928.

Test Plan:
Run with --logdir gs://tensorboard-bench-logs/edge_cgan to verify that
RustBoard still works. Then, dump the ldd output:

$ cd tensorboard/data/server/
$ cargo build --release
$ ldd target/release/rustboard
	linux-vdso.so.1 (0x00007ffe8afbd000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f422533e000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f422531c000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f42251d8000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f42251d2000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f422500d000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f4225b66000)

…and verify that all the shared libraries are on the manylinux2010
whitelist.

wchargin-branch: rust-rustls

[wchargin]

@psybuzz: Would you mind giving this a sanity check on macOS? I think
that it should be pretty safe—this removes a system-level dependency
on OpenSSL, and rustls/ring don't link against system libraries as
far as I can tell. But it would be nice to double-check.

The following should suffice:

bazel run //tensorboard -- --load_fast true --logdir gs://tensorboard-bench-logs/edge_cgan --bind_all

[psybuzz]

Checked that with this PR, running bazel run //tensorboard -- --load_fast true --logdir gs://tensorboard-bench-logs/edge_cgan --bind_all on MacOS 11.3 works properly; scalars and images load fine.

[wchargin]

Thank you!