Seedtool Needs to Deal with Unknown Data

Seedtool needs to have a methodology for dealing with unknown data in Envelopes of seeds it imports.

For example:
```
% envelope format ur:envelope/lrtpcsgdhkwzdtfthptokigtvwnnjsqzcxknsktdoyadcsspoycfadzttpcstantjyoeadisktjojeisdefzdydtaolytantjloxaxhdclaxvlcprfttldjobkredtlnhsidwybaeyjtswyandlgjnehtkdsidbkqzsrkphyfhsaaahdcxhnfgnepefxgdytryckticelyotsstoknfntavevaskiddmolsarntykbrybtjpksamtantjooeadlncsghykaeykaeykaocyhngrmuwzaycyzssajpsnoybdtpcskpfyhsjpjecxgdkpjpjojzihcxgdihiajecxhfinhsjztkwewyfn
Bytes(16) [
'isA': 'Seed'
'508': output-descriptor(Map)
'hasName': "Dark Purple Peck Vial"
]
```
Currently if you import this into Seedtool and reexport it, the output-descriptor is silently dropped.
```
% seedtool -i envelope ur:envelope/lrtpcsgdhkwzdtfthptokigtvwnnjsqzcxknsktdoyadcsspoycfadzttpcstantjyoeadisktjojeisdefzdydtaolytantjloxaxhdclaxvlcprfttldjobkredtlnhsidwybaeyjtswyandlgjnehtkdsidbkqzsrkphyfhsaaahdcxhnfgnepefxgdytryckticelyotsstoknfntavevaskiddmolsarntykbrybtjpksamtantjooeadlncsghykaeykaeykaocyhngrmuwzaycyzssajpsnoybdtpcskpfyhsjpjecxgdkpjpjojzihcxgdihiajecxhfinhsjztkwewyfn -o envelope | envelope format
Bytes(16) [
'isA': 'Seed'
'hasName': "Dark Purple Peck Vial"
]
```
Options for dealing with this:

1. Silently drop it [current]
2. Drop it with a *WARNING*
3. Silently carry it over
4. Carry it over with a *WARNING*
5. Drop it, but offer an additional Envelope containing only the dropped data.

...

There are potentially three categories of data here, with the possibility to deal with them differently.

A. Known data types (like output-descriptor)
B. Unknown data types
C. Attachments

复现文档中记录的 seedtool Envelope 导入/导出命令，并将导入的输出与原始 Envelope 进行比较。跟踪 Envelope 处理的入口点，然后确定对于已知数据类型、未知数据类型和附件的预期行为；完成的标准是达成一致的策略，并确保 output-descriptor 数据不再被静默丢失。

由索引模型根据 Issue 内容生成。