changed extra_bootconfig_args to vector for multy vm support

[adelva1984]

No description provided.

[Databean]

extra_bootconfig_args is a free-text string, this now imposes the requirement that it cannot contain any commas. That may break some existing callers.

The strategy we used to deal with some other free-text flags for multi-instance vectorization was to instead add a new base64-encoded version of the flag, which could then be separated by commas.

In fact, we already have extra_bootconfig_args_base64_vec defined:

android-cuttlefish/base/cvd/cuttlefish/host/commands/assemble_cvd/assemble_cvd_flags.h

Line 106 in 73094a2

DECLARE_vec(extra_bootconfig_args_base64);