BREAKING: Restrict Custom identifiers to c / cN and add OptableVID - #63

Why

OptableIdentifier.Custom(key, value) sent key:value for any key. The iOS SDK only allows the custom types c and c1…cN (case custom(Int? = nil, String)), with the Optable visitor ID as its own optableVID case, and that behaviour was confirmed as the correct one. This brings Android in line.

What Changed

• android_sdk: breaking Custom(key: String, value) is replaced by Custom(index: Int?, value) and Custom(value). A null or 0 index is sent as c, any other as c<abs(index)>, matching iOS. The value is trimmed, as before.
• android_sdk: new OptableIdentifier.OptableVID(value) for the Optable visitor ID, previously Custom("v", value). Sent as v, whitespace removed, as before.
• android_sdk: IdentifiersEncoder builds the c / cN prefix from the index; no other prefix can come from Custom. Raw is unchanged.
• android_sdk tests: moved to the new API, plus a test for no index, null, 0 and a negative index.
• README: identifier and hids examples use OptableVID, Custom(value) and Custom(index, value).

How to Test

• ./gradlew test passes (161 tests locally)
• Verified in DemoAppJava / DemoAppKotlin (the demos don't use Custom)
• README updated (if public API changed)

Notes

Do not merge before v1.3.0 is tagged, or the break ships in 1.3.0. This is for 2.0.0: optableVersionName in gradle.properties needs bumping to 2.0.0 (separately, as #52 did for 1.3.0).

Migration: Custom("v", x) → OptableVID(x); Custom("c", x) → Custom(x); Custom("cN", x) → Custom(N, x). Any other key is no longer supported; use Raw("type:value") if it is really needed.

After this merges, #60's identifier table needs its Custom row updated and an OptableVID row added.

• Breaking change
• Requires release

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