WIP: Fix missing index handling fieldcaps

[smalyshev]

DO NOT MERGE: this is an experimental pull to see what happens, not meant to be actually committed.

Attempted fix for the ESQL missing indices issue (see #126275 and #114495).

This is done by using accept_unavailable=false on fieldcaps. This is NOT meant to be merged as is, just to expose how much exactly this would break if we change the ignore_unavailable option.

[smalyshev]

Currently broken tests:

EsqlSecurityIT » testLookupJoinIndexForbidden
EsqlAsyncSecurityIT » testFromLookupIndexForbidden
EsqlSecurityIT » testUnauthorizedIndices
EsqlAsyncSecurityIT » testUnauthorizedIndices
EsqlSecurityIT » testInsufficientPrivilege
EsqlSecurityIT » testFromLookupIndexForbidden
EsqlAsyncSecurityIT » testLookupJoinIndexForbidden
EsqlAsyncSecurityIT » testInsufficientPrivilege
FailureStoreSecurityRestIT » testFailureStoreAccess
CrossClusterEsqlRCS1MissingIndicesIT » testSearchesAgainstNonMatchingIndices
RemoteClusterSecurityEsqlIT » testSearchesAgainstNonMatchingIndices
RemoteClusterSecurityEsqlIT » testCrossClusterQueryWithOnlyRemotePrivs
RemoteClusterSecurityEsqlIT » testAlias

RequestIndexFilteringIT » testIndicesDontExist
RestEsqlIT » testUseUnknownIndex {SYNC}
EsqlRestValidationIT » testInexistentIndexNameWithoutWildcard
RestEsqlIT » testUseUnknownIndex {ASYNC}
EsqlRestValidationIT » testInexistentIndexNameWithoutWildcard
CrossClusterQueryWithFiltersIT » testFilterWithMissingRemoteIndex
CrossClusterQueryWithFiltersIT » testFilterWithUnavailableFieldCapsAndSkipU...
CrossClusterQueryWithFiltersIT » testFilterWithMissingIndex
RequestIndexFilteringIT » testIndicesDontExist
RestEsqlIT » testUseUnknownIndex {SYNC}
RestEsqlIT » testUseUnknownIndex {ASYNC}