Fix http header accept parsing problem (#3185)

## Issue Addressed

Which issue # does this PR address?
#3114 

## Proposed Changes

1. introduce `mime` package 
2. Parse `Accept` field in the header with `mime`

## Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.

common/eth2/Cargo.toml

@@ -26,6 +26,7 @@ futures-util = "0.3.8"
 futures = "0.3.8"
 store = { path = "../../beacon_node/store", optional = true }
 slashing_protection = { path = "../../validator_client/slashing_protection", optional = true }
+mime = "0.3.16"
 
 [target.'cfg(target_os = "linux")'.dependencies]
 psutil = { version = "3.2.2", optional = true }