Source code refactoring. Extracted common functionality in cpp_helpers.c...

* Fix json ignore unknown

Previously, there were several problems with ignoring unknown in json.
1) After finding a field is unknown, the parser's state is not changed. Thus, there is no way to distinguish whether the parser is dealing with an unknown field or it's just a top level message.
2) Several method didn't respect unknown field, e.g., start_object, end_bool, start_array.

* Update json parser size

* Update json parser size

Before, every charAt would emit (on android):
```
    0x00002104    adrp x17, #+0x1000 (addr 0x3000)
    0x00002108    ldr w17, [x17, #20]
    0x0000210c    ldr x0, [x0, #128]
    0x00002110    ldr x0, [x0, #328]
    0x00002114    ldr lr, [x0, #24]
    0x00002118    blr lr <-- Call into String.charAt(int)
```
Now, it emits the inlined implementation of charAt (branch is for possibly compressed strings):
```
    0x000020b4    ldur w16, [x4, #-8]
    0x000020b8    tbnz w16, #0, #+0xc (addr 0x20c4)
    0x000020bc    ldrb w4, [x4, x0]
    0x000020c0    b #+0x8 (addr 0x20c8)
    0x000020c4    ldrh w4, [x4, x0, lsl #1]
```

PiperOrigin-RevId: 590821750

Before, every charAt would emit (on android):
```
    0x00002104    adrp x17, #+0x1000 (addr 0x3000)
    0x00002108    ldr w17, [x17, #20]
    0x0000210c    ldr x0, [x0, #128]
    0x00002110    ldr x0, [x0, #328]
    0x00002114    ldr lr, [x0, #24]
    0x00002118    blr lr <-- Call into String.charAt(int)
```
Now, it emits the inlined implementation of charAt (branch is for possibly compressed strings):
```
    0x000020b4    ldur w16, [x4, #-8]
    0x000020b8    tbnz w16, #0, #+0xc (addr 0x20c4)
    0x000020bc    ldrb w4, [x4, x0]
    0x000020c0    b #+0x8 (addr 0x20c8)
    0x000020c4    ldrh w4, [x4, x0, lsl #1]
```

PiperOrigin-RevId: 590821750

Before, every charAt would emit (on android):
```
    0x00002104    adrp x17, #+0x1000 (addr 0x3000)
    0x00002108    ldr w17, [x17, #20]
    0x0000210c    ldr x0, [x0, #128]
    0x00002110    ldr x0, [x0, #328]
    0x00002114    ldr lr, [x0, #24]
    0x00002118    blr lr <-- Call into String.charAt(int)
```
Now, it emits the inlined implementation of charAt (branch is for possibly compressed strings):
```
    0x000020b4    ldur w16, [x4, #-8]
    0x000020b8    tbnz w16, #0, #+0xc (addr 0x20c4)
    0x000020bc    ldrb w4, [x4, x0]
    0x000020c0    b #+0x8 (addr 0x20c8)
    0x000020c4    ldrh w4, [x4, x0, lsl #1]
```

PiperOrigin-RevId: 591147406

…lbuffers#128)