crypto_provider: avoid rand dep for GREASE HPKE

[cpu]

Previously we used the rand crate as a convenient way to select a supported HPKE suite at random for ECH when using GREASE. In general it's crummy to take a dep for such a minor need.

Instead, adapt the crypto provider CSRNG to this task by performing a truncated uniform sampling. This should be more than sufficient for the use-case and avoids an extra dep.

Resolves #529

[ctz]

for my own interest, i checked this fixed #529 and it did:

 readelf --wide --symbols target/x86_64-unknown-linux-gnu/release/deps/librustls.so  | rustfilt | grep -e chacha
   271: 000000000010ddc0    31 FUNC    LOCAL  DEFAULT   12 aws_lc_rs::cipher::key::SymmetricCipherKey::chacha20
  1640: 0000000000048860     6 FUNC    LOCAL  DEFAULT   12 aws_lc_rs::aead::chacha::init_chacha_aead
  2707: 00000000000dc640     6 FUNC    LOCAL  DEFAULT   12 aws_lc_rs::aead::chacha::init_chacha_aead
  2939: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS chacha.c
  2941: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS e_chacha20poly1305.c
  2942: 000000000010fee0     5 FUNC    LOCAL  DEFAULT   12 aead_chacha20_poly1305_cleanup
  2945: 0000000000110080   121 FUNC    LOCAL  DEFAULT   12 aead_chacha20_poly1305_init
  2946: 0000000000110100   995 FUNC    LOCAL  DEFAULT   12 chacha20_poly1305_seal_scatter
  2947: 00000000001104f0    61 FUNC    LOCAL  DEFAULT   12 aead_chacha20_poly1305_seal_scatter
  2948: 0000000000110530   504 FUNC    LOCAL  DEFAULT   12 aead_chacha20_poly1305_open_gather
  2949: 000000000039f680    96 OBJECT  LOCAL  DEFAULT   22 aead_chacha20_poly1305
  3013: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS 564c5311b313cdb2-chacha20_poly1305_x86_64.o
  3014: 000000000015ebc0     0 NOTYPE  LOCAL  DEFAULT   12 chacha20_poly1305_constants
  3016: 0000000000163200  9753 FUNC    LOCAL  DEFAULT   12 chacha20_poly1305_open_avx2
  3017: 0000000000165840 12205 FUNC    LOCAL  DEFAULT   12 chacha20_poly1305_seal_avx2
  3877: 000000000010fd20   443 FUNC    LOCAL  DEFAULT   12 aws_lc_0_24_0_CRYPTO_chacha_20
  3993: 0000000000160f80  8770 FUNC    LOCAL  DEFAULT   12 aws_lc_0_24_0_chacha20_poly1305_seal
  4187: 0000000000110730    12 FUNC    LOCAL  DEFAULT   12 aws_lc_0_24_0_EVP_aead_chacha20_poly1305
  4969: 0000000000087340     6 FUNC    LOCAL  DEFAULT   12 aws_lc_rs::aead::chacha::init_chacha_aead
  5491: 000000000015edc0  8583 FUNC    LOCAL  DEFAULT   12 aws_lc_0_24_0_chacha20_poly1305_open
  6368: 000000000010d560   128 FUNC    LOCAL  DEFAULT   12 <aws_lc_rs::cipher::chacha::ChaCha20Key as core::ops::drop::Drop>::drop
  6492: 000000000010c310   133 FUNC    LOCAL  DEFAULT   12 aws_lc_rs::aead::aead_ctx::AeadCtx::chacha20