Add optional support for AEGIS encryption

AEGIS is a new family of authenticated encryption algorithms that offers
stronger security, higher usage limits, and better performance than AES-GCM.

This pull request adds support for a new `-aegis` command-line flag, allowing
AEGIS-128X2 to be used as an alternative to AES-GCM on CPUs with AES acceleration.

It also introduces the ability to use ciphers with different key sizes.

More information on AEGIS is available here:
- https://cfrg.github.io/draft-irtf-cfrg-aegis-aead/draft-irtf-cfrg-aegis-aead.html
- https://github.com/cfrg/draft-irtf-cfrg-aegis-aead

gocryptfs -speed speed on Apple M1:

AES-GCM-256-OpenSSL              3718.79 MB/s
AES-GCM-256-Go                   5083.43 MB/s   (selected in auto mode)
AES-SIV-512-Go                    625.20 MB/s
XChaCha20-Poly1305-OpenSSL       1358.63 MB/s   (selected in auto mode)
XChaCha20-Poly1305-Go             832.11 MB/s
Aegis128X2-Go                   11818.73 MB/s

gocryptfs -speed speed on AMD Zen 4:

AES-GCM-256-OpenSSL              5215.86 MB/s
AES-GCM-256-Go                   6918.01 MB/s   (selected in auto mode)
AES-SIV-512-Go                    449.61 MB/s
XChaCha20-Poly1305-OpenSSL       2643.48 MB/s
XChaCha20-Poly1305-Go            3727.46 MB/s   (selected in auto mode)
Aegis128X2-Go                   28109.92 MB/s

1 files changed, 2 insertions, 0 deletions

diff --git a/go.sum b/go.sum
index 3e4cb01..e92d87f 100644
--- a/go.sum
+++ b/go.sum
@@ -1,3 +1,5 @@
+github.com/aegis-aead/go-libaegis v0.2.10 h1:5BJd8dgl2Fv3XwJ+W1dSWoS9FBaA61ryLnu2E4H2Vdw=
+github.com/aegis-aead/go-libaegis v0.2.10/go.mod h1:WKSG0pcdvpDrwDPBl9NdAIxHxpZOCqlKWH7EOnPLKHc=
 github.com/aperturerobotics/jacobsa-crypto v1.0.2 h1:tNvVy1rev9FagnOyBmTcI6d23FfNceG9IujZROTRtlc=
 github.com/aperturerobotics/jacobsa-crypto v1.0.2/go.mod h1:buWU1iY+FjIcfpb1aYfFJZfl07WlS7O30lTyC2iwjv8=
 github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=