[FEATURE] make target deletion (drop) tolerate failures

[georgeh0]

When we delete (drop) a target backend, sometimes it fails because the connection is no longer available. Currently this may block flow's setup process.

We want to make target deletion (drop) tolerate failures. Specifically:

• Add a new field ignore_target_drop_failures to Settings. Use env variable COCOINDEX_IGNORE_TARGET_DROP_FAILURES as the default value.
• When ignore_target_drop_failures is set to true, when there's any error in dropping a target backend, log an error instead of popping up the error which terminates the process.
• When ignore_target_drop_failures is set to false, in the error message created during dropping a target backend, put a hint to let users know about the COCOINDEX_IGNORE_TARGET_DROP_FAILURES option.

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[prithvi-moonshot]

@georgeh0 Hi, I can take this

[badmonster0]

assigned, thanks!