Option to override ALPNs

[backkem]

Currently one can add a additional custom ALPN to the list of supported ALPNs in the QuicVersionManager. However, ALPNs that are based on the supported Quic version are always available.

quiche/quiche/quic/core/quic_version_manager.h

Lines 50 to 54 in eac20c9

	// Subclasses may add custom ALPNs to the supported list by overriding
	// RefilterSupportedVersions() to first call
	// QuicVersionManager::RefilterSupportedVersions() then AddCustomAlpn().
	// Must not be called elsewhere.
	void AddCustomAlpn(const std::string& alpn);

Consider the following use-case: I want to build an application protocol, using Quic v1 as the transport protocol. However, I want to use a custom ALPN instead of h3 since this server would not support HTTP. This doesn't seem possible to have a list consisting of only the custom ALPN now.
In my specific case, I'd like to have a list that only contains "osp" as defined here.

Would this be amenable? If so, would it be feasible to break up QuicVersionManager::RefilterSupportedVersions or would adding a ClearSupportedAlpns be preferred?