feat (processors.execd): introduce buffer_size config option

### Relevant telegraf.conf

```toml
[[inputs.netflow]]
metric_buffer_limit = 500000
flush_interval = "60s"
service_address = "udp4://:2055"
protocol = "ipfix"
private_enterprise_number_files = ["telegraf_conf/netflow_ingestion/custom_fields.csv"]
dump_packets = false

[[processors.execd]]
command = ["telegraf_conf/netflow_ingestion/netflow_options_packet_parser.py"]
namepass = ["netflow_options"]
buffer_size ="1Mib"
```

### Logs from Telegraf

```text
2024-07-29T23:56:51Z E! error loading config file telegraf_conf/netflow_ingestion/telegraf_netflow copy.conf: plugin processors.execd: line 70: configuration specified the fields ["buffer_size"], but they were not used. This is either a typo or this config option does not exist in this version.
```

### System info

1.32.0

### Docker

_No response_

### Steps to reproduce

1. add buffer_size ="1Mib" to your telegarf file and run it
2.
3.
...

### Expected behavior

We'd like a similar option to what is seen here https://github.com/influxdata/telegraf/blob/release-1.31/plugins/inputs/execd/README.md the reason for this ask is due to stderr messaged we need to log out are quite large, this is to assist us with debugging

### Actual behavior

2024-07-29T23:56:51Z E! error loading config file telegraf_conf/netflow_ingestion/telegraf_netflow copy.conf: plugin processors.execd: line 70: configuration specified the fields ["buffer_size"], but they were not used. This is either a typo or this config option does not exist in this version.

### Additional info

We are wanting to increase the stderr buffer to show more verbose logs
atm we are seeing
2024-07-29T23:58:50Z E! [processors.execd] Error reading stderr: bufio.Scanner: token too long

Start at the processors.execd entry point and compare its stderr handling with the inputs/execd README linked in the issue, focusing on the existing buffer_size behavior. Confirm the expected configuration format and identify the relevant processor tests; done means the option is accepted and larger stderr output no longer fails with bufio.Scanner: token too long.

Written by the indexing model from the issue text.