implement GEP-1742 timeouts in the policy controller

[hawkw]

PR #10969 adds support for the GEP-1742 timeouts field to the
HTTPRoute CRD. This branch implements actual support for these fields in
the policy controller. The timeout fields are now read and used to set
the timeout fields added to the proxy-api in
linkerd/linkerd2-proxy-api#243.

In addition, I've added code to ensure that the timeout fields are
parsed correctly when a JSON manifest is deserialized. The current
implementation represents timeouts in the bindings as a Rust
std::time::Duration type. Duration does implement
serde::Deserialize and serde::Serialize, but its serialization
implementation attempts to (de)serialize it as a struct consisting of a
number of seconds and a number of subsecond nanoseconds. The timeout
fields are instead supposed to be represented as strings in the Go
standard library's time.ParseDuration format. Therefore, I've added a
newtype which wraps the Rust std::time::Duration and implements the
same parsing logic as Go. Eventually, I'd like to upstream the
implementation of this to kube-rs; see kube-rs/kube#1222 for details.

Depends on #10969
Depends on linkerd/linkerd2-proxy-api#243