Previously, set_ech_keys would consume the SslEchKeys struct to enforce
the requirement that the struct is immutable after initializing it on a
SSL_CTX. The problem with this is that it requires applications to
needlessly reallocate the SslEchKeys struct if they want to initialize
keys on multiple SSL_CTXs, which is a pretty common pattern. To work
around this, we introduce a builder (SslEchKeysBuilder) that requires
mutable access to add keys to the underlying struct. set_ech_keys takes
in a reference to SslEchKeys, which can only be made via consuming the
builder.
Kornel