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MINOR: [Java] add toString on AbstractContainerVector (#40404)

### Rationale for this change

Adding the ability to see the values in vectors derived from AbstractContainerVector (e.g. structs, dense unions) when something's not quite going as you're expecting :slightly_smiling_face:  

### What changes are included in this PR?

Add AbstractContainerVector.toString in line with BaseValueVector.toString

### Are these changes tested?

Yes, as part of our XTDB development, but not in the Arrow repo - I didn't see any other tests for ValueVector.toString implementations?

### Are there any user-facing changes?

Strictly speaking, yes, if the user's printing out the vector - but Object.toString isn't often considered part of the public API in my experience?

Cheers!

James

Authored-by: James Henderson <james@jarohen.dev>
Signed-off-by: David Li <li.davidm96@gmail.com>