Proxy Protocol Added Tlv Value Encoding Option

Added :ref:encoding <envoy_v3_api_field_extensions.filters.listener.proxy_protocol.v3.ProxyProtocol.KeyValuePair.value_string_encoding> option to control how a TLV value is encoded before it is stored in dynamic metadata or filter state. By default the TLV value is sanitized to a valid UTF-8 string (previous behavior). Setting it to BASE64 stores the raw TLV value as a base64-encoded string instead.