Add ECDSA signing support for CloudFront URLs

[luigi617]

Issue #, if available:

Description of changes:
Support ECDSA encryption algorithms in CloudFront signed URLs.

Testing:
Generate test keys

TMP=$(mktemp -d)
openssl ecparam -name prime256v1 -genkey -noout -out "$TMP/ec_sec1.pem"        # EC SEC1
openssl pkcs8 -topk8 -nocrypt -in "$TMP/ec_sec1.pem" -out "$TMP/ec_pkcs8.pem"  # EC PKCS#8
openssl genrsa -out "$TMP/rsa.pem" 2048                                        # RSA (regression)
openssl ecparam -name secp384r1 -genkey -noout -out "$TMP/ec_p384.pem"         # not supported

Sign

python -m awscli cloudfront sign \
  --url https://d1.example.com/x --key-pair-id K123 \
  --private-key file://"$TMP/<output_pem>" --date-less-than 2030-01-01

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[GarrettBeatty]

can you put testing details in PR description with how you tested?

[luigi617]

added