colbuilder: clean up type schema handling

This commit refactors how we're keeping track of the current type schema
of the operators in `NewColOperator`. Previously, we would create a new
type slice for each operator due to "type schema corruption" bugs we
observed (#47889). We fixed that bug by being extremely conservative,
and this commit applies a different more reasonable fix.

In particular, it is safe to append to the current type slice we have in
scope, and we only need to be careful when we're trying to create
a "projection" (i.e. when we need to change the order of types or modify
one type in-place). Thus, this commit switches to making a copy only in
those scenarios which should happen at most once per processor spec
(previously, it could happen thousands of times for elaborate render
expressions).

Furthermore, this commit reuses the same type slice from `InputSyncSpec`
since creation of the operators occurs _after_ the spec has been
communicated across the wire (or locally), so we're free to use it as we
please.

```
name                               old time/op    new time/op    delta
NestedAndPlanning/renders=16-24       627µs ± 1%     624µs ± 2%     ~     (p=0.143 n=10+10)
NestedAndPlanning/renders=256-24     3.54ms ± 0%    3.04ms ± 1%  -14.14%  (p=0.000 n=9+10)
NestedAndPlanning/renders=4096-24     211ms ± 4%      68ms ± 1%  -67.61%  (p=0.000 n=10+10)

name                               old alloc/op   new alloc/op   delta
NestedAndPlanning/renders=16-24      74.0kB ±20%    68.9kB ±10%     ~     (p=0.053 n=10+9)
NestedAndPlanning/renders=256-24     1.71MB ± 0%    0.60MB ± 0%  -65.07%  (p=0.000 n=8+8)
NestedAndPlanning/renders=4096-24     303MB ± 0%      13MB ± 1%  -95.58%  (p=0.000 n=8+8)

name                               old allocs/op  new allocs/op  delta
NestedAndPlanning/renders=16-24         754 ±18%       733 ±18%     ~     (p=0.105 n=9+9)
NestedAndPlanning/renders=256-24      6.44k ± 0%     5.93k ± 0%   -7.88%  (p=0.000 n=8+8)
NestedAndPlanning/renders=4096-24      146k ± 6%      136k ± 0%   -7.02%  (p=0.000 n=8+8)
```

Release note: None